In a conventional petroleum or petrochemical refinery process and system, crude feedstock is processed by a crude distillation unit. The crude feedstock may comprise crude oil and/or feedstock having undergone partial processing (“intermediate refinery feedstock”). The crude distillation unit produces a naphtha fraction, together with a number of other fractions useful in production of refined oil products, for example, gasoline, jet fuel, diesel, etc., and fractions useful for the production of specialty chemicals.
The naphtha fraction is primarily composed of paraffins, olefins, naphthenes and aromatics. Paraffins are alkane hydrocarbons of general formula CnH2n+2, which may be substituted, and wherein n is a whole number; e.g., from 1-14. The term “paraffins” is also generally understood to include isoparaffins. Olefins are hydrocarbons having at least one carbon-carbon double bond, such as an alkenes of general formula CnH2n, which may be substituted and wherein n is a whole number; e.g., from 2-14. The olefin fraction may also comprise alkynes of general formula CnH2n−2, which may be substituted and wherein n is a whole number; e.g., from 2-14. When n is greater than 12 and less than 21, the fraction may be referred to as distillates; e.g., jet fuel, diesel, etc. Higher n fractions may be useful for other purposes. Olefins (including substituted olefins) where n=12-14 may be found in both the naphtha fraction and the distillates fraction. The naphthenes include cycloalkanes and alkyl substituted cycloalkanes. Many naphthenes are chemical precursors to the aromatics. The aromatics found in a petroleum or petrochemical feedstock include a range of conjugated hydrocarbon rings and alkyl substituted conjugated hydrocarbon rings.
Hydrocarbon fractions are often referred to as Cn fractions or Cn+ fractions with n being a whole number. It is to be understood that Cn+ includes the nth fraction (i.e., the Cn fraction) as well as higher n fractions.
The whole range naphtha fraction from the crude distillation unit is processed in a naphtha splitter producing an overhead stream (typically referred to as a Light Straight Run or LSR), and a bottoms stream of heavy naphtha. The LSR is rich in paraffins, and the heavy naphtha is rich in naphthenes and aromatics. The heavy naphtha bottoms stream is hydrotreated to remove sulphur and other contaminants, obtaining a sweet naphtha, which is fed to a naphtha reformer where it may be combined with other intermediate sweet naphtha streams, for example, sweet natural gas condensates and hydrocracker naphtha. In the naphtha reformer, the naphtha components are reformulated into components of a gasoline product.
The naphtha reformer is usually a high severity reformer, which produces aromatics, including benzene, toluene and xylenes (“BTX”), as well as other aromatics that enable the reformate to have an octane quality sufficient to meet gasoline octane specifications. Benzene, toluene and xylenes may all also be used in the production of petrochemical derivatives. Of the xylenes that may be used in the production of petrochemical derivatives, para- and ortho-xylene are worth particular mention, although meta-xylenes may also be of value.
High severity reformers are run at high temperatures (e.g., inlet temperatures of about 900 degrees Fahrenheit—around 480 degrees Celsius—or greater) with commercially available catalyst, and have long residence times. The residence time is a factor of the number of reactors and the amount of catalyst involved. High severity reformers typically involve four or five reactors in series. Also, at each reactor, heating to the noted inlet temperature is required in order to produce the desired gasoline products. High severity reformers are associated with high operating costs and may result in a significant volume loss of high economic value gasoline components.
Gasoline is a blend of LSR and reformate and other gasoline components obtained from the crude, such as butane, alkylate, isopentane, methyl tertiary butyl ether, ethanol and catalytic cracker gasoline. The proper boiling point, octane number, and other gasoline specifications are met by the blend of the LSR, reformate and other gasoline components.
The benzene content of gasoline has been regulated to a low value in many nations including Canada and the United States. Refiners have chosen four methods to reduce benzene in the gasoline product in order to produce a gasoline having an octane quality sufficient to meet gasoline octane specifications. Refiners have: (1) removed benzene precursors before the naphtha reformer to preclude or reduce benzene production in the reformer; (2) hydrotreated or saturated the benzene fraction of the reformate to convert the benzene to non-aromatics; (3) developed new reformer catalysts that selectively do not react with the benzene precursors; and (4) removed benzene from the reformate or a fraction thereof by aromatics extraction. With the exception of the aromatics extraction method, each of these methods reduce the net amount of benzene available for benzene derivative production thereby increasingly moving the industry to more expensive methods to produce benzene.
Aromatics extraction is most commonly performed by solvent or extractive distillation. C6 to C8 aromatics are fractionated in a fractionation tower, usually into benzene and a toluene/xylenes mixture. The raffinate stream obtained after extraction of these aromatics can be blended into gasoline. Additional benzene could be produced from the toluene and xylenes recovered by fractionation using hydrodealkylation and/or toluene disproportioning processes. Producing benzene from these two processes introduces additional capital cost and volume loss of expensive feedstock.
The term “raffinate” is used throughout this application. It will be understood by the skilled reader that the composition of the raffinate will depend on the specific extraction process, and feedstock.
Alternative sources of benzene, such as pyrolysis gasoline (“pygas”), catcracker gasoline, and Coker naphtha are also available, but obtaining benzene from these sources involve high operating costs. Also, these sources, although significant in volume, are not sufficient to satisfy the world benzene market.
Another high cost process for producing benzene is the UOP/BP Cyclar process that uses propane and butane to make benzene and other aromatics. There is a world scale unit at a British Petroleum (BP) refinery in the United Kingdom. This process has the limitation of not producing a significant portion of the aromatics as benzene, thus not adequately satisfying the world benzene market.
A lower cost method to produce benzene directly in the refinery process would be beneficial.
A process and system for extraction of chemical components from a feedstock, such as petroleum, natural gas condensate or petrochemical feedstock, that maximizes high economic value streams at high volumetric yields would also be of benefit.
Attempts have been made to modify the conventional refinery process, such as by fractionating feedstock and using different reformers on each fraction, see, for example, U.S. Pat. No. 6,051,128.